In recent years, a semiconductor element such as a field-effect transistor is formed on an SOI (Silicon On Insulator) substrate (Patent document 1). The SOI substrate is generally constituted by a stacked structure comprising a Si (silicon) substrate, an insulation layer (SiO2 layer) deposited on the Si substrate, and a crystalline Si layer further deposited on the insulation layer.
Such an SOI substrate can realize both of the high speed processing capability and low power consumption of LSI (Large Scale Integrated Circuit) and the like, and has drawn attention as a next semiconductor substrate. In the SOI substrate, a Si layer has been currently thinned (15 nm or less) progressively so as to decrease the parasitic capacitance of a MOS transistor or the like.
In recent years, the following problem on the SOI substrate has been raised which has occurred along with the tendency of thinning the Si layer.
When the outermost Si layer of the SOI substrate is approximately 10 nm thick and is heated and temperature-raised according to some circumstances, the Si layer causes a phenomenon of agglomeration on a certain temperature condition. An example of such a heating and temperature-raising process is the case of growing a Si epitaxial film on the outermost Si layer of the SOI substrate through the heating and temperature-raising process. In this case, a cohering action occurs in the above described Si layer, which causes a problem that the Si epitaxial film is hardly grown on the Si layer.
In addition, another example is the case of forming a film on the Si layer by using a CVD film-forming process. In this case, it is known that when the film-forming step includes a process of raising the temperature to approximately 600° C. by heating, agglomeration occurs in the Si layer in a stage of the heating and temperature-raising process.
The reason why the agglomeration occurs in the Si layer as was described above is assumed to be because when the Si layer becomes as thin as approximately 10 nm, the thickness of the Si layer corresponds to that of several atomic layers and then it becomes difficult to keep the state of the film layer stable. However, the exact reason why the agglomeration occurs in the Si layer is not sufficiently elucidated in the present stage.
Then, Patent document 1 is directed to the problem of the above described agglomeration of the Si layer. In the method for manufacturing the semiconductor device according to Patent document 1, two methods are proposed in order to prevent the agglomeration of the Si layer.
The first method is a method of controlling the heat treatment temperature to a temperature lower than the agglomeration temperature of the Si layer, when the surface of the Si layer is exposed in heat-treating a substrate having the Si layer formed thereon in a non-oxidizing atmosphere. (Patent document 1, paragraph [0009] and the like). However, there is a case in which the substrate needs to be temperature-raised to the agglomeration temperature or higher. In this case, the temperature is raised by covering the surface of the Si layer with a protective film, which is referred to as a second method. In the second method, a protective film of SiO2, Si3N4 or the like is formed on the surface of the Si layer, before the Si layer is heat-treated at the agglomeration temperature or higher. (Patent document 1, paragraph [0010] or the like).
By the way, when the Si epitaxial layer is grown on the outermost Si layer of the SOI substrate, for instance, as was described above, the Si epitaxial layer needs to be temperature-raised to the agglomeration temperature of the Si layer or higher, and the surface of the Si layer needs to be exposed. In such a case, the method described in Patent document 1 cannot solve the problem of the above described agglomeration.
Patent documents 1: Japanese Patent Application Laid-Open No. 2002-353426